To be honest I dont even notice any gaps top or bottom of the chimney on my first engine so I wouldn't worry about filling it. It gets so hot it might not stand up to the heat any way and it also gets muck on it after a few steams. For the top I have used 4 3/32 copper rivets at 90 deg spacing to keep the cap in place. Can take a pic if you wish, did the same on the first engine.

Trust me unless you're like the guy in last months Old Glory ( who I have met and is a very nice bloke) you wont be able to keep it spotless once you get going with it, besides if its too clean it just doesnt look right, even Tims engine is very tidy but still has a "worked " look to it which it should have.

On the ones I've done I've always bedded the base ring and the copper top in araldite around the chimney. I then use Araldite it to fill any gaps where the steel overlaps. I've never had any of it come loose or burn away. I don't clean the inside of the chimney too often (you have to keep it clear around the blast nozzle & blower) so the layer of oily sooty sludge acts as a pretty good insulator (probably!). When it gets about 6mm thick I use a angled piece of waste pipe to peel off the sludge from th etop down, works a treat. Bye the way if you ever want to black yourself up for some reason then the gunge from your chimney is perfect! but a bu**er to get off.